The present invention is directed to an acid-stable porous sheet product which has a structural configuration suitable for forming an improved battery separator, to an improved battery separator formed from said separate sheet product and to a battery, in particular to a lead-acid battery, which has the present improved separator located between adjacent plates of opposite polarity.
Storage batteries have been known and used for over a century. A conventional storage battery is formed of one or more units or cells, each of which has a positive electrode, a negative electrode, separator elements between electrodes of opposite polarity and an electrolyte, such as aqueous sulfuric acid.
Separators for a lead-acid battery should prevent contact between electrodes of opposite polarity yet permit contact between electrode and excess electrolyte to produce efficient electrochemical reactions. For example, electrodes formed from lead and lead oxide must be maintained in contact with an excess of sulfuric acid electrolyte solution to permit the double sulfate reaction to occur between the electrode's material and the acid during discharge while also providing sufficient ionic strength electrolyte for ion transfer. Further, separators should permit the removal of gaseous products which are formed on the plate surfaces during the charging operation of any charge/discharge cycle. Oxygen is normally formed and evolved at the positive plates and hydrogen at the negative plates. The gaseous products are generally in the form of minute bubbles and may form at any point on an electrode plate surface. These bubbles are impediments to electrolyte/electrode contact required to have efficient battery operation.
In early battery designs, where compactness and energy density were not a prime objective, electrode elements of opposite polarity were maintained sufficiently apart by separator pins or posts which readily allowed the flow of electrolyte and egress of formed gaseous products.
In a modern storage battery, a group of interconnected plates forming the positive electrode is intermeshed with another group of interconnected plates which act as the negative electrode to give alternating positive/negative plate orientation within each cell of the battery. Each plate must be maintained apart from adjacent plates of opposite polarity by some form of separation means. Contact may be due to imperfections in plate structure or due to warping or wrinkling of the plate which may occur during assembly or during normal operation of the battery as well as by other chemical or physcial phenomena. The desire to develop batteries of compact, high specific capacity (electrical energy/unit weight) in which the separator membranes are sandwiched between electrodes of opposite polarity requires a separator which is capable of being formed from thin, light weight sheet material; providing means for the removal of gases; providing for electrode/electrolyte contact within a minimum spacing; and exhibiting the ability to withstand the compressive forces encountered from adjacent electrode plates. Such forces tend to distort and sometimes permanently collapse the gaseous egress means designed in some coventional separators. The separator component is recognised as a key element in forming a highly efficient battery.
Prior art separators used in lead-acid storage batteries have included various designs which provide spacer arrangements such as separators which are laminated with glass mat or which have ribs or projections formed on at least one major surface. Such separators are costly to form and add material and weight to the battery system. Further, separators which have glass mats as part of their structure have the defect of permitting gas bubbles to be lodged and retained within the mat's fiberous structure. Useful ribbed separators have heretofore only been formed from thick sheet stock since thin sheet materials have been found too flexible to permit effective alignment of the separator between adjacent electrode plates of opposite polarity.
Separators with spacing arrangements have also been formed from embossed sheet products. Embossment of various designs are known and include straight corrugated configurations, such as described in U.S. Pat. No. 2,662,106, or projections, as described in U.S. Pat. Nos. 2,382,829; 2,465,493; 4,072,802; and 4,153,759. The known corrugated type of separator does not have structural integrity when formed from thin sheet stock. Thin corrugated separators have the defect of succumbing to the compressive forces and collapsing against a plate surface. Separators of straight corrugated design are suitable for gas release only when formed from rigid and thicker than desired stock. Separators having embossed separate projections, such as shown in U.S. Pat. Nos. 2,382,829 and 2,465,493, tend to trap gaseous products in their individual cavities. Separators such as shown in U.S. Pat. Nos. 4,072,802 and 4,153,759, are capable of being formed from thin sheet stock, but have conical projections which tends to block the egress of gaseous products. U.S. Pat. No. 4,228,225, is directed to a separator capable of being formed from thin sheet material and provides an embossed configuration having continuous vertically orientated gas egress paths. The egress paths of the separator described in '225 have constricted areas which may retard the removal of gas from the battery. The presently described sheet product enhances and promotes the removal of formed gaseous products by a design which forms non-constricted egress paths while providing the other desired properties.
An object of the present invention is to provide a sheet product suitable for use as a battery separator.
A further object of the present invention is to provide a thin sheet product suitable as a battery separator which has paths on both sides of the battery separator to provide for and enhance the removal of gaseous products formed within the battery.
A further object of the present invention is to provide a thin sheet product suitable for use as a battery separator which provides means for enhancing the removal of gaseous products while also providing substantial electrolyte to electrode contact to form an effective battery.
A still further object of the present invention is to provide a thin sheet product suitable for use as a battery separator which has means to provide for and enhance the removal of gaseous products, to provide for good electrolyte to electrode contact and to have sufficient resistance to mechanical forces encountered to maintain its configuration during use.
A still further objective of the present invention is to provide a thin porous sheet product suitable for use as a battery separator wherein the sheet has a first and a second major face with each face containing a plurality of separate continuous open channels such that each channel on one face defines a separation between two channels on the other face, and that some of the channels have a configuration which has some degree of lateral extension beyond a minimum lateral extension of the channel with respect to an imaginary median line for that channel, every channel has at least one continuous channel defining contour line formed from straight line segments and substantially every portion of each channel defining contour line is oriented at an acute angle of no greater than 70 degrees from the vertical.
Another object of the present invention is to provide a porous sheet which has substantially uniform thickness throughout; having a configuration which provides improved gaseous egress means on both major surfaces and which can be formed into a wrap-around or enveloping configuration while maintaining the same improved gaseous egress means in said wrap-around or U configuration.
Another object of the subject invention is to provide a sheet product suitable for use as a battery separator wherein the sheet has a first and a second major face with each major face containing a plurality of separate continuous open channels such that each channel on each face has a configuration which has some degree of lateral extension beyond a minimum lateral extension with respect to an imaginary median line for the channel, every channel has at least one continuous channel defining contour line formed from straight line segments such that substantially every portion of each channel defining contour line as viewed on the face is orientated at an acute angle of no greater than 70 degrees from the vertical.
A further object of the present invention is to provide an acid-stable porous battery separator from a sheet product of this invention.
A further object of the present invention is to provide an acid battery having a container, an electrolyte at least one pair of electrode plates of opposite polarity and a separator formed from a sheet product of this invention positioned between and at least coextensive with each pair of electrode plates.